<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    * {
        padding: 0;
        margin: 0;
        list-style: none;
        box-sizing: border-box;
    }

    .box {
        width: 300px;
        height: 200px;
        border: 1px solid #000;
        margin: auto;
    }

    .box>.head {
        display: flex;
        justify-content: space-between;
    }

    .box>.head>li {
        flex-grow: 1;
        text-align: center;
        cursor: pointer;
    }

    .active {
        background-color: #87ceeb;
    }

    .main {
        position: relative;
    }

    .main>li {
        position: absolute;
        top: 0;
        left: 0;
        width: 100%;
        height: 178px;
        background-color: #87ceeb;
    }

    .mainActive {
        z-index: 999;
    }
</style>
</head>
<body>
  <div class="box">
    <ul class="head">
        <li class="active">标题一</li>
        <li>标题二</li>
        <li>标题三</li>
    </ul>

    <ul class="main">
        <li class="mainActive">内容一</li>
        <li>内容二</li>
        <li>内容三</li>
    </ul>
    <script>
        //获取所有的li
        let li1 = document.querySelectorAll(".head li");
        let li2 = document.querySelectorAll(".main li");
        for(let i = 0;i < li1.length;i++){
            li1[i].onclick = function(){
                for(let j = 0;j < li1.length;j++){
                    li1[j].className='';
                    li2[j].className='';
                }
                li1[i].classList.toggle('active');
                li2[i].classList.toggle('mainActive');
            }
            
        }
    </script>
</div>
</body>
</html>